Horton

Re: New strategies for freezing, advancing relfrozenxid early

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Andres Freund <andres@anarazel.de>, Jeff Davis <pgsql@j-davis.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, John Naylor <john.naylor@enterprisedb.com>, Justin Pryzby <pryzby@telsasoft.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-01-26T20:54:17Z
Lists: pgsql-hackers 
On Thu, Jan 26, 2023 at 2:57 PM Peter Geoghegan <pg@bowt.ie> wrote:
> Relatively difficult for Andres, or for somebody else? What are the
> real parameters here? Obviously there are no clear answers available.

Andres is certainly smarter than the average guy, but practically any
scenario that someone can create in a few lines of SQL is something to
which code will be exposed to on some real-world system. If Andres
came along and said, hey, well I found a way to make this patch suck,
and proceeded to describe a scenario that involved a complex set of
tables and multiple workloads running simultaneously and using a
debugger to trigger some race condition and whatever, I'd be like "OK,
but is that really going to happen?". The actual scenario he came up
with is three lines of SQL, and it's nothing remotely obscure. That
kind of thing is going to happen *all the time*.

> The overwhelming cost is usually FPIs in any case. If you're not
> mostly focussing on that, you're focussing on the wrong thing. At
> least with larger tables. You just have to focus on the picture over
> time, across multiple VACUUM operations.

I think that's all mostly true, but the cases where being more
aggressive can cause *extra* FPIs are worthy of just as much attention
as the cases where we can reduce them.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

Commits

  1. Revert "Add eager and lazy freezing strategies to VACUUM."

  2. Add eager and lazy freezing strategies to VACUUM.

  3. Refine the definition of page-level freezing.

  4. Avoid special XID snapshotConflictHorizon values.

  5. Add page-level freezing to VACUUM.

  6. Remove overzealous MultiXact freeze assertion.

  7. Refactor how VACUUM passes around its XID cutoffs.

  8. Deduplicate freeze plans in freeze WAL records.

  9. Set relfrozenxid to oldest extant XID seen by VACUUM.

  10. Only skip pages marked as clean in the visibility map, if the last 32

  11. Add vacuum_freeze_table_age GUC option, to control when VACUUM should